Chart Update Conflicting with a Notice to Mariners
You are preparing a chart update for a coastal approach. Your new survey shows a least depth of 6.8 m over a shoal previously charted at 7.4 m. A Notice to Mariners issued three weeks ago reports a temporary buoy marking a wreck 1.5 nm away, and a local harbour authority has verbally told you that dredging is planned but not yet approved. The chart is due for a new edition in six weeks. Your survey data are complete and validated, but the wreck position in the Notice to Mariners is given only to the nearest 0.1 minute of latitude and longitude.
1. How do you handle the shoal depth change and the wreck in the chart update, given the differing data quality?
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Shoal: The new least depth of 6.8 m is from validated survey data and should be charted, with the source and date noted. The change from 7.4 m to 6.8 m is a significant shoaling and should be highlighted in the accompanying report and, if warranted, a preliminary notice.
Wreck: The Notice to Mariners position is only to 0.1 minute (roughly 185 m in latitude, less in longitude), which is insufficient for precise charting. Do not plot the wreck as a precise point. Instead:
- Chart the wreck as a temporary/approximate feature consistent with the Notice, or
- Reference the Notice in the chart's notes and defer precise plotting until a better position is available.
Distinction: Survey data (high confidence) vs. notice data (low positional confidence) must be treated differently; do not let the weaker data degrade the chart's integrity.
2. How do you treat the unapproved dredging plan, and what do you communicate to the harbour authority?
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Treatment: Unapproved plans are not charted. Charts depict the current state of the seabed and navigational features, not proposed works. Charting a dredged depth that does not yet exist would be a serious safety error.
Communication:
- Confirm in writing with the harbour authority that the dredging is not yet approved and request the approved design depth and dates if/when it is.
- Note in the chart report that dredging is planned but not reflected in the chart.
- If dredging is imminent, recommend a preliminary notice and a future chart update rather than pre-empting the change.
Record the verbal advice and your response in the project file to protect against later disputes.